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  1. #1
    Join Date
    May 2013
    Posts
    44
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    One or Two stage equipment

    Virginia climate, Zone 4, 3-ton, 80K

    What makes the most sense for a new system in this area? To me simple=dependability but sacrifices efficiency.

    Furnace 80% or 90+% Single stage multi-speed or two stage variable speed, PSC or EMC

    AC Condenser single stage or two, PSC or EMC

    How about single stage AC with two stage furnace or vice versa?

    Currently looking at a 96% single stage with single stage AC but SEER and EER is lower. Also hard to find this combo with an AHRI number.

    Don't need the best just want a good system that doesn't require fixing every year. I understand EMC efficiencies and replacement costs just not sure if I should be concerned with that since the warranties are all 10yrs.

    Thanks

  2. #2
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    9,240
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    Single speed anything, long term is better. Changing air flow also changes distribution patterns, making balancing more difficult.
    One benefit from ECM fan motors is fine tuning the ideal setup for the a/c. The ideal speed of blower determines the amount dehumidification you get while removing sensible heat from the home. Most green grass climates have excess moisture adequate fresh air ventilation/infiltration plus the moisture from the occupants during low/no sensible cooling during high outdoor dew points. Most live with occasional high indoor humidity during this kind of weather. The first line of defense is to setup the a/c to max dehumidification during peak sensible cooling loads to slightly over-dry the materials in the home during peak cooling. The ECM fan motor more speeds available.
    Also some like to have constant air movement in the home to make the indoor temperatures more consistent 24/7. This is the biggest advantage of the ECM motor. <100 watts of electricity gets a very quiet and efficient 80-100 cfm of air circulation. This is not possible with a PSC fan motor.

    Two speed a/c does a slightly better at removing moisture during partial cooling loads by avoid some on/off cycles during part load conditions. But can not provide real %RH control during low/no sensible cooling times with high outdoor dew points plus the moisture from the occupants. This would evenings and rainy days. An occasional rainy day may be acceptable but a rainy week ends up with home start to grow mold and dust mites.
    The ideal fix for this is a simple a/c with an ECM motor and a small whole house dehumidifier. Then you can set the temperature and %RH you want and you have total control. Units like the Ultra-Aire whole house dehumidifiers connect to the home and a/c providing dehumidification when the a/c is unable to remove enough moisture. Cost a little more but makes home/basement/crawlspace home more comfortable in all green grass climates.
    Hope this helps.
    Regards Teddy Bear
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  3. #3
    Join Date
    May 2013
    Posts
    44
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    Thread Starter
    It helps, thanks.

  4. #4
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    39,766
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    You likely won't find an AHRI rating for a PSC furnace with an A/C. That would be what AHRI calls a coil only rating. The furnace does nothing to change the SEER. But if you took SOME 14 SEER units, for example, and put them on an ECM furnace, you could rate out at 15 SEER so they publish those matches.

    Personally I don't see a big advantage of 2 stage. Longer run time but sometimes due to reduced capacity on a big coil, the dehumidifying capability on low is less so you may not gain anything. And as the bear says, if it isn't on, such as cool damp evening, you got nothing.

    I'd get a good single stage unit and put the money you save into one of the bear's whole house dehumidifiers. Then you will have a dry house!

  5. #5
    Join Date
    May 2013
    Posts
    44
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    Thread Starter
    Thanks. Think I found an excellent installer and decided to go single stage for both with a 96% furnace. Current ducts barely sufficient so we are adding additional return and supply. Have not had any humidity problems except maybe being too dry in winter.

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